The surveillance was discontinued at some point last year for lack of evidence, according to one of the sources.
The FBI then restarted the surveillance after obtaining a new FISA warrant that extended at least into early this year.
Sources say the second warrant was part of the FBI’s efforts to investigate ties between Trump campaign associates and suspected Russian operatives. Such warrants require the approval of top Justice Department and FBI officials, and the FBI must provide the court with information showing suspicion that the subject of the warrant may be acting as an agent of a foreign power.
It is unclear when the new warrant started. The FBI interest deepened last fall because of intercepted communications between Manafort and suspected Russian operatives, and among the Russians themselves, that reignited their interest in Manafort, the sources told CNN. As part of the FISA warrant, CNN has learned that earlier this year, the FBI …
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